1. Field of the Invention
This invention relates to detectors having a thin film of boron nitride (BN) such as cubic BN and methods, systems and arrays utilizing same.
2. Background Art
The following references are related to the invention and may be referenced herein:    1. G. F. Knoll, “Radiation detection and measurement,” 3 edition, Wiley, 2000.    2. See, Broad Area Announcement on Nuclear Materials and Security, Department of Homeland Security, Federal Register, 2006.    3. D. Litvinov and Roy Clarke, “Reduced bias growth of pure-phase cubic boron nitride,” Appl. Phys. Lett. 71, 1969-1971 (1997).    4. S. Kidner, C. A. Taylor and Roy Clarke, “Low-Energy Kinetic Threshold In The Growth Of Cubic Boron-Nitride Films,” Appl. Phys. Lett. 64, 1859-1861 (1994).    5. D. Litvinov, C. A. Taylor and Roy Clarke, “Semiconducting cubic boron nitride,” Diamond and Related Materials 7, 360-364 (1998).    6. D. Litvinov and Roy Clarke, “In Situ Texture Monitoring for Growth of Oriented Cubic Boron Nitride Films,” Appl. Phys. Lett. 74, 955-957 (1999).
Miniaturized solid-state detectors have a very broad range of applications, both civilian and military [1]. In particular, personal radiation monitoring devices, covert radiation monitoring, border inspections, detection of illicit trafficking of nuclear materials, and nuclear power industry personnel protection, are all areas that can benefit from the proposed technology. Neutron detection is a key element in all border control strategies [2].
The technical difficulty in achieving an efficient neutron detector comes from the way neutrons interact with matter. From this point of view, they are fundamentally different than alpha, beta and electromagnetic radiation. These three types of radiation are categorized as ionizing radiation because they produce ion-electron pairs as they travel through matter. Alpha and beta types of radiation consist of electrically charged particles; they can easily interact (via Coulomb forces) with the electrons in the atomic layers and transform them to free carriers. Electromagnetic radiation also interacts with the electron shells to a greater or lesser extent, depending on the photon energy. Once free carriers are generated, they are usually collected on electrodes with the help of an electric field created by the means of an applied bias voltage.
In contrast, neutrons are normally detected by participating in nuclear reactions. Compared with the other types of radiation mentioned above, this is a fundamentally different type of interaction. In order to translate the result of this type of interaction into a measurable electrical signal, an intrinsic mechanism must be used to transfer energy between the nucleus and the atomic electronic shells. Commonly, detector applications utilize other particles that result from neutron capture reactions to achieve this goal: charged particles (alpha) or gamma photons can trigger avalanche carrier generation (in the case of detectors) or can produce another form of detectable signal (visible light, in the case of scintillating layers).
Neutrons are relatively difficult to observe because, in general, they only interact with select nuclei and the interaction probability is small. Since the interaction medium normally has a low density (liquid or gas), the usual approach to increase the overall interaction probability is to increase the neutron path inside the material.
Current neutron based nuclear material detection is based mainly on gas-filled neutron capture chambers. An immediate consequence is that the size of the detector increases accordingly, this being the reason why state of the art neutron detectors are currently large and cumbersome.
Boron (isotope 10) has a high capture cross-section for thermal neutrons and has long been used (mostly in gaseous form) in nuclear detection technology. Of all isotopes likely to interact with thermal neutrons, 10B has three properties that make it extremely attractive from the detector application point of view: a) the reaction yields a very energetic alpha particle that can produce copious charge carriers in the surrounding material:
n + 10B→7Li + 4He(2.3 MeV) + γ(0.48 MeV)(93%)→7Li + 4He(2.8 MeV)(7%)
Because of its electrical charge, the alpha particle interacts easier with the surrounding material to produce charge carriers. The most common reaction path (93%) also yields a 0.48 MeV gamma photon. However, because boron has a small number of electrons, the number of carriers produced by the gamma photon is negligible compared to that produced by the alpha particle; b) 10B has among the largest probability of interaction with the thermal neutrons, described by the neutron capture cross section of 3837 barn [1]; and c) the natural abundance of isotope 10B is relatively large (˜19.7%), making it relatively inexpensive.
Solid state neutron detectors are disclosed in U.S. Pat. No. 6,921,903 (which utilizes CdZnTe-based material) and U.S. Pat. No. 6,727,504 (which describes detectors based on bulk (hexagonal/pyrolytic) form of BN).
In the design described in U.S. Pat. No. 6,727,504 an electric field is applied across a slab of BN (hexagonal, in pyrolytic form) and the signal produced by the alpha particles is recorded in the form of a current peak. Since h-BN is highly insulating, the collection of the charge carriers produced by the neutron capture event has a very low efficiency.
U.S. Pat. Nos. 5,940,460 and 5,969,359 disclose a diode that works in conjunction with a neutron conversion layer. The neutron passes through the neutron conversion layer and, if it is absorbed by a nucleus (B, Li etc.), will produce an alpha particle that can be detected by the semiconductor diode. In other words, the neutrons are converted into alpha particles in a conversion layer outside the semiconductor device. A similar situation is met in x-ray detection where photons can be converted into light in a phosphor and the light converted into an electrical signal in a diode—an approach referred to as indirect detection. A drawback comes from the fact that the neutrons are absorbed outside the devices, again reducing the overall efficiency. A significant fraction of the alpha particles (approximately half) will have momentum pointing in a direction away from the diodes. Another disadvantage is the charge reabsorption outside the detection element (the diode). The thicker the conversion layers, the more absorbed neutrons but the thicker the layers, the more alpha particles get reabsorbed.
Other relevant patent documents include: U.S. Pat. Nos. 5,334,840; 6,479,826; 6,545,281; 6,624,423; 6,771,730; 7,034,307; 7,164,138; 7,372,009; U.S. Publication 2006/0291606; and WO 2007/109535.